1. Field of the Invention
The present invention relates to systems and methods for satellite navigation, and in particular to a system and method for reducing error from beacon measurements used for satellite navigation, and for reducing payload pointing error.
2. Description of the Related Art
Spacecraft typically have one or more payloads that are directed to transmit or receive energy from ground stations. For example, communication satellites include one or more uplink antennas for receiving information from an uplink center, and one or more downlink antennas for transmitting and/or receiving (transceiving) information with terrestrial transceivers. The uplink and downlink antennas are typically disposed on the satellite body (or spacecraft bus) and are directed toward a terrestrial location where an uplink/downlink antenna is transmitting/receiving the information.
In many cases, the information is beamed to and/or received from a plurality of terrestrial receivers spanning a wide geographical area. In such situations, the pointing accuracy of the uplink/downlink antennas are not particularly critical. However, in other cases, spacecraft payloads must be pointed at the desired target with a high degree of accuracy. This can be the case, for example, in cases where the uplink/downlink antenna is a narrow beamwidth antenna, or when spatial diversity is critical. In such situations, a spacecraft's on-board navigation system (which relies on inertial sensors and perhaps Sun, Earth, Moon, star, and magnetic sensors as well) often cannot support the precise pointing requirement.
In such cases, beacon sensor systems can be used to increase payload pointing performance and spacecraft body attitude accuracy. The beacon sensor system monitors an uplink carrier (which can also be used to provide commands to the satellite) to sense mispointing of the antenna structure. Using the beacon sensor data as a reference, the satellite navigational system parameters can be updated to improve accuracy. The beacon sensor data can be used to replace other sensor data.
Recent technology advances include the use of digital beacons. In a digital beacon, the beacon beams are formed digitally using an on-board Digital Signal Processor (DSP). The beacon beams are formed by selecting desired beam weights for each feed chain. However, the accuracy of the digital beacon system is negatively affected by the performance limitations of the digital beam-forming technique and its implementation. Although some digital beacon sensor errors can be ameliorated by calibration and the adjustment of weighting to beacon sensor channels (beamweights), asymmetry errors due to beam-forming approximation by finite number of feed chains, quantization errors due to the finite-bit representation of the weighting factors themselves, and errors in the gain and phase calibration of each of the beacon sensor channels can severely impact beacon accuracy and therefore payload pointing accuracy. What is needed is a system and method for compensating for such asymmetry error and quantization errors. The present invention satisfies this need.